Vehicles known from common practice are, in each case, designed with at least one transmission system between a drive input arrangement and a drive output, in order to obtain various operating ranges such as forward drive, reverse drive, and thrust operation or traction operation. The drive torque provided by the drive input arrangement is directed to the front axle in front-wheel-drive vehicles, to the rear axle in rear-wheel-drive-drive vehicles, and in all-wheel-drive vehicles to two or more driven vehicles axles with various degrees of distribution.
In these vehicles the transmission systems are in each case designed with a main transmission with which various gear ratios can be produced. Such main transmissions are in the form of gearshift transmissions, variable-speed automatic transmissions or even infinitely-variable automatic transmissions, which can be operated both in an automatic shifting mode and in a manual shifting mode. During the automatic shifting mode, a gear ratio is selected automatically depending on the current driving condition of the vehicle. During the manual shifting mode, a gear ratio can be selected on the basis of a desire indicated by the driver.
A change between the automatic shifting mode and the manual shifting mode can be commanded by a driver by means of a shift lever or a switch, for example one arranged in the area of a steering wheel, whereby actuation of the shift lever or switch can in addition also trigger a demand for a gear ratio change starting from an actual gear ratio currently engaged in the main transmission toward a target gear ratio. In this, an upshift or a downshift is commanded by virtue of a so-termed tip-minus or a tip-plus function.
After transitioning to the manual shifting mode or to a so-termed short-term manual mode, time monitoring is commenced and it is checked whether, within a defined time period, further shift commands have been issued by the driver. If this is not the case, then provided that various boundary conditions are fulfilled, preferably after the predefined time period has lapsed, the system automatically changes back again to the automatic shifting mode.
If there is a command by the driver for a downshift, for example because of a current driving situation to increase a currently available traction force provision or to increase a thrust torque of a engine in the form of an internal combustion engine in the area of a drive output, and if the downshift command is carried out by the so-termed tip-minus function at the same time as a change from the automatic shifting mode to the manual shifting mode, then in the known procedures, starting from the actual gear ratio a simple downshift to the next-lower gear ratio is authorized by the tip-minus function or, in the case of infinitely-variable transmissions, the gear ratio value currently set is adjusted continuously in the direction of a larger gear ratio.
At present variable-speed automatic transmissions are increasingly being produced with a larger number of gear ratio steps, with correspondingly smaller step intervals. Some gear ratios are so-termed overdrive gears by means of which vehicles can be operated over large operating ranges with low engine running speeds. In such cases vehicles with internal combustion engines, designed in the area of their drivetrains with decoupling means such as turbine torsion dampers, can be operated with very low engine running speeds.
With vehicle concepts of that type, a gear ratio change that is triggered by the tip-minus function sometimes does not produce the increase of traction force or thrust torque in the area of the drive output that is expected by the driver by virtue of his shift command, or there is no significant change of the torque applied at the drive output.
Starting from the above-described operating points of a drivetrain of a vehicle characterized by low running speeds of the engine, to produce a significant increase of the traction force or thrust torque it is necessary for the driver to command several consecutive downshifts, which in certain driving situations, such as when initiating a spontaneous overtaking process, cannot be carried out as spontaneously as necessary.
To assist the driver, shift levers or switches have been functionally improved to the effect that by actuating the shift lever or switch continuously over a longer period in the range of the tip-minus function, as the operating time increases further consecutive downshifts are triggered. However, the operating time within which a sometimes desired or expected torque at the drive output is made available for a driver, is made only very slightly shorter thereby.
When a downshift in a transmission system is commanded by the driver, then according to U.S. 2009/0171539 A1 a target gear ratio should be selected from a limited range of gear ratios compared with the total number of gear ratios available in the transmission system, in order to produce an increase of the torque applied at a drive output within acceptable operating times. The result of this is that when the driver commands a downshift starting from a gear ratio step currently engaged in the transmission, sometimes a multiple downshift is carried out. It is proposed, for example that in a transmission having eight gear ratio steps, starting with the eighth gear step currently engaged but also starting with the seventh gear step currently engaged, in both cases a driver's downshift command brings about a downshift to the sixth gear ratio step. This means that a driver's downshift command issued when the eighth gear is engaged triggers a downshift through two gear ratio steps, whereas if the driver's downshift command is issued when the seventh gear is engaged, a downshift by only one gear step takes place.
Disadvantageously, when selecting the gear ratio step to be engaged in the transmission system when the driver issues a downshift command, the current operating condition of the transmission system is not taken into account, and for that reason, during unfavorable operating condition variations of a vehicle drivetrain made with the transmission system concerned, the downshift does not result in the torque change in the area of the drive output that the driver expects.
To produce a change of the drive output traction force, called for by a driver's downshift command, with the spontaneity expected by the driver, according to the procedure described in U.S. 2009/0171540 A1 after the driver has commanded a downshift, a gear ratio step is selected, which differs by a minimum gear ratio from the currently engaged gear ratio step. Depending on the layout of the gear ratios of a transmission system and the choice of the gear ratio change for a downshift, a driver's downshift command starting from the current gear ratio step can in some cases result in a multiple downshift.
In this procedure too, when a downshift is commanded the existing operating status of the transmission is disadvantageously not considered in the selection of the target gear to be engaged.
Furthermore, from U.S. 2009/0171542 A1 it is known, when the driver commands a downshift, to select from the total number of gear-ratios available in a transmission system, a limited range of gears depending on the driving speed, the road inclination or a steering angle, from which the target gear to be engaged on the basis of the driver's downshift command can be selected.
During driving operation of a vehicle, when there is a downshift command and for example the currently engaged gear ratio step is the fourth gear or the currently engaged gear ratio step is the fifth gear at the same vehicle speed, a double downshift to the second, or to the third gear ratio step, respectively, will be carried out regardless of the current operating status.
Accordingly, the driving behavior of a vehicle characterized by the torque applied in the drive output area after the gear ratio change concerned cannot be anticipated by the driver.